THE PRESENT INVENTION relates to a shredding machine for shredding sheet material. The present invention relates particularly, but not exclusively, to a shredding machine in the form of a paper-shredder suitable for home or office use.
Over recent years it has been customary to provide shredding machines in domestic homes or work places such as offices, in order to provide a convenient method of securely disposing of confidential documentation or other sensitive papers.
Conventional paper shredders of the type mentioned above are provided with a paper feed-aperture, particularly in the form of a feed-slot of elongate form, through which a plurality of paper sheets or the like can be fed towards a pair or rotating cutters located below the feed-slot which serve to shred the paper sheets into a plurality of strips having a width of only a few millimeters, the resulting strips of paper being collected in a basket or bin located below the cutters. For reasons of space and economy, the cutting mechanisms used in conventional paper shredders of this type are only effective in shredding stacks of paper or card up to a relatively small predetermined thickness. If a stack of papers or cards exceeding this predetermined thickness is inserted into the feed-slot, for example by being force-fed into the slot by an over-enthusiastic user, it is possible to present the shredding mechanism with such a bulk of material so as to overload the mechanism and stall the driving motor or otherwise jam the mechanism. Not only can paper-jams of this type represent an annoyance to a person using the paper shredder, but they can serve to damage the cutting mechanism, for example by distorting the shafts of the cutters or damaging the cutting blades.
In co-pending International Patent Application PCT/GB06/004286, the applicants have disclosed an anti-jam mechanism to prevent overloading of a paper shredder by inserting sheet material of too great a thickness in the manner described above. The shredding machine of PCT/GB06/004286 comprises a feed passage extending from a feed aperture and further comprises a cutting mechanism driven by an electric motor, the feed aperture and feed passage being configured to receive multiple sheets and to direct said sheets towards the cutting mechanism for shredding. This machine is provided with an actuating element part of which extends into the feed passage and which is movable from a first position in which the actuating element permits energisation of the cutting mechanism, past a second position beyond which the actuating element prevents energisation of the cutting mechanism. The actuating element is biased towards its first position and is arranged to actuate a switch when moved past said second position, to break the electrical circuit providing power to the cutting mechanism. The shredding machine of PCT/GB06/004286 thus has a threshold thickness of superimposed sheets such that the machine will not attempt to shred a stack of superimposed sheets if the stack has a thickness above that threshold, herein referred to as the anti-jam threshold.
The applicants have found, however, that the machine of PCT/GB06/004286 suffers from the following problems, in common with prior art shredders without the anti-jam system of PCT/GB06/004286, namely:
Where the shredder is powered from a main supply, there is the difficulty that mains supply voltage is variable, within a certain tolerance, with the result that the maximum sheet capacity, in practice, of the mains driven electrical shredder will be less when the mains voltage is at the lower end of its tolerance range than when the voltage is at the higher end of that range.
The temperature of the electric motor driving the shredder rises during use, causing the motor to be less efficient after a period of use, producing a drop in output power and hence a drop in sheet capacity.
During the life of the shredder, the cutting unit and transmission system wear and become less efficient, the cutting mechanism clogs with paper dust and lubrication dries out or wears off, all of which place a greater load on the motor, again resulting in a drop in sheet capacity.
In view of the above factors, the applicants found it necessary to set the anti-jam threshold, i.e. the thickness threshold at which the actuating mechanism operated to prevent energisation of the cutting mechanism, at a “worst-case” level and thus significantly below the actual cutting capacity of the cutting mechanism under conditions better than the “worst case” set of conditions.